1. Field of the Invention
The present invention relates to an optical element and an optical device and, more particularly, to an optical device such as a projection type image display device or an optical information processing device using a spatial light modulator.
2. Description of the Related Art
As a large-screen image display device, a large-scale cathode ray tube has been developed. However, in order to meet a demand for a larger screen, a projection type display device has received a lot of attention. As the projection type display device, a projection type CRT display device which projects and displays an image displayed on a compact high-definition, high-luminance compact CRT, and a projection type liquid crystal display device which projects and displays light modulated by a liquid crystal panel are commercially available. In particular, the latter device is suitable for a compact, lightweight structure, and can be easily applied to domestic equipment.
FIG. 1 shows the typical arrangement of a conventional projection type liquid crystal display device. Referring to FIG. 1, light emitted by a high-luminance light source 1 such as a metal halide lamp is reflected by a reflector 2 to be collimated or converging light with a collimation, and the light is irradiated onto a liquid crystal panel 3. The light transmitted through the liquid crystal panel 3 is imaged and projected onto a screen 5 via a projection lens 4.
The projection method includes two types, i.e., a rear projection type for projecting an image from the rear side of the screen when viewed from an observer, and a front projection type for projecting an image from the same side as an observer. When a color image is to be displayed, a single-panel type device using a liquid crystal panel with three color filters (e.g., R (red), G (green), and B (blue) filters), a three-panel type device using three liquid crystal panels corresponding to the three color components, and the like are used. The three-panel type projection type liquid crystal display device includes a device which uses a dichroic mirror and the like to split light emitted by a light source into three color components, and projects these color components three projection lenses, and a device which synthesizes the split color components again using a dichroic mirror, and the like, and projects the synthesized light using a single projection lens.
In addition to the above-mentioned device using light transmitted through the liquid crystal panel, a reflection type display device which uses light reflected by a liquid crystal panel has also been developed. As the matrix type of the liquid crystal panel, a simple matrix, a thin film transistor, a thin film diode, and the like are available, and as the operation mode of a liquid crystal, various modes such as the TN mode, the STN mode, and the like are available. Recently, a system for controlling scattering and transmission of light using a polymer dispersed type liquid crystal has also been developed. In addition, a display system which uses an element having micro reflector arrays formed in a process similar to the manufacturing process of semiconductor integrated circuits has been developed.
FIG. 2 shows another arrangement of a conventional projection type liquid crystal display device. This device is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 6-175129. This device improves the utilization efficiency of light by improving the convergence of light emitted by a light source to attain an improved contrast ratio and uniformity of a projected image, thus achieving a compact, strong structure.
Referring to FIG. 2, a light source 11 is disposed at the first focal point position of an spheroidal mirror 12. A conical prism 13 is disposed near the second focal point position of the spheroidal mirror 12. A light beam which is emitted by the light source 11 and is focused by the spheroidal mirror 12 is adjusted by the prism 13 to eliminate a dark area formed by the shadow of the light source 11 itself, and is focused at the central portion. The light beam transmitted through the prism 13 is projected onto a screen (not shown) via a collimator 14, a transmission type display element 15, a field lens 16, an aperture stop 17, and a projection lens 18. After the light beam is adjusted by the convex or concave surface of the prism, and is converted into a collimated beam by the collimator 14, the light beam is incident on the transmission type liquid crystal display element 15.
However, the above-mentioned projection image display device using a spatial light modulator such as a liquid crystal panel has a feature in that a large-screen display can be relatively easily attained, but it is difficult to assure sufficient brightness. Therefore, this device requires a dark room, and also requires large consumption power since a large-output lamp is used as a light source. In particular, the ratio of the power of light which reaches the screen of the light which is emitted by the light source, i.e., the utilization efficiency of light is as low as 1 to 2%. For this reason, an improvement of the utilization efficiency of light in the respective portions of an optical system from an illumination system to the screen via a color-separation system, a spatial modulating system (liquid crystal panel), a color synthesis system, and the projection lens is an important problem.
In particular, in the arrangement shown in FIG. 1, since the intensity distribution of the light beam has the shadow of the light source 1 itself at the central portion, a dark shadow appears at the center of the screen 5, thus posing a problem associated with the nonuniformity of brightness. In the arrangement shown in FIG. 2, the shadow of the central portion is eliminated by focusing light to the central portion using the conical prism 13. However, in the arrangement shown in FIG. 2, since the bulk-shaped prism 13 is disposed near the focal point of the light beam propagating from the spheroidal mirror 12, the light power is locally concentrated in a portion of the prism 13, and the shape of the prism tends to be thermally distorted, thus often causing destruction of the prism. In order to avoid this problem, when the position of the prism 13 is separated from a position near the focal point of the light, the utilization efficiency of the light further deteriorates. In order to prevent this, the area of the incident surface of the prism 13 must be increased, and as a result, the size, in the optical axis direction, of the prism 13 also increases, thus disturbing realization of a compact device.